1. Field of the Invention
The present invention relates to a high-frequency superposing module for driving a laser diode by superposing a high-frequency current on a DC current.
2. Description of the Related Art
In a conventional laser-diode drive circuit, a laser diode is driven in a multiple mode where a high-frequency current is superposed on a driving DC current, thereby preventing the generation of mode-hopping noise resulting from an increase in the temperature of the laser diode. For superposing a high-frequency current on a DC current, an oscillating circuit that generates a high-frequency signal is used as described in, for example, Japanese Unexamined Patent Application Publication Nos. 2002-251763 and 10-256630 and Domestic Re-publication of PCT Publication No. 98/01856.
FIG. 7 is a circuit diagram of a conventional laser-diode drive circuit disclosed in Japanese Unexamined Patent Application Publication No. 10-256630. In this circuit, a DC current is supplied via an input terminal LDD to a laser diode LD. A low-pass filter composed of capacitors C7 and C8 and inductors L4 and L5 is disposed between the terminal LDD and the laser diode LD. The rest of the circuit shown in FIG. 7 is an oscillating circuit section for superposing a high-frequency current for driving the laser diode LD. The oscillating circuit section is supplied with power via a power input terminal Vcc. Resistors R1 to R4 apply a predetermined DC bias to a transistor Q1. This oscillating circuit section is a Colpitts oscillator and its oscillation frequency is determined by the values of capacitors C1, C2, C4, C5, and C9 and inductors L1 and L2. Furthermore, the capacitors C4 and C5 act as an impedance matching circuit section between the laser diode LD and the oscillating circuit section.
FIG. 8 shows the main part of a circuit described in an embodiment of Japanese Unexamined Patent Application Publication No. 2002-251763. In FIG. 8, resistors R1 to R3 apply a predetermined DC bias to a transistor Q1. This oscillating circuit section is also a Colpitts oscillator whose oscillation frequency is determined by the values of capacitors C1, C2, and C3 and inductors L1 and L2. An inductor L3 represents a load impedance and draws an oscillation output for a laser diode LD from the collector of the transistor Q1.
Referring back to FIG. 7, a typical circuit requires two power supplies: one for supplying a DC current to the laser diode LD and the other for the oscillating circuit section which generates a high-frequency current to be superposed on the DC current. The less the number of power supplies, the more preferable the circuit is. Likewise, the less the number of external terminals, the more preferable the circuit is for compact design of a module, particularly if the oscillating circuit section is implemented in a different module from that for the laser diode, that is, if the oscillating circuit section is implemented as a separate module for use in combination with the laser diode.
The object described above is achieved in the circuit shown in FIG. 8, where the high-frequency signal output terminal of the oscillating circuit section also functions as a power input terminal.
Unfortunately, the circuit shown in FIG. 8 cannot have an impedance matching circuit section unlike the circuit in FIG. 7, because the circuit in FIG. 8 requires the power input terminal, which also functions as the output terminal of the oscillating circuit section, to be connected directly to the laser diode to pass a DC current, thus preventing a capacitor from being disposed in series in the output portion of the oscillating circuit section.